U.S. patent application number 15/538949 was filed with the patent office on 2017-12-14 for electrolyte for lithium secondary battery and lithium secondary battery comprising same.
The applicant listed for this patent is Samsung SDI Co., Ltd.. Invention is credited to Jung Joo CHO, Jong Ho JEON, Sung Nim JO, Jin Hee KIM, Joo Hwan KOH, Tae Hwan YU.
Application Number | 20170358826 15/538949 |
Document ID | / |
Family ID | 56150890 |
Filed Date | 2017-12-14 |
United States Patent
Application |
20170358826 |
Kind Code |
A1 |
KOH; Joo Hwan ; et
al. |
December 14, 2017 |
ELECTROLYTE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY
BATTERY COMPRISING SAME
Abstract
An electrolyte solution for a lithium secondary battery includes
a lithium salt, an organic solvent, and a solid salt as an
additive, the solid salt including at least one cation selected
from ammonium-based cations and an azide anion (N.sub.3--). Using
the electrolyte solution including the additive may provide a
lithium secondary battery with improved high-temperature retention
characteristics.
Inventors: |
KOH; Joo Hwan; (Seoul,
KR) ; JEON; Jong Ho; (Daejeon, KR) ; KIM; Jin
Hee; (Suwon-si, KR) ; JO; Sung Nim; (Seoul,
KR) ; YU; Tae Hwan; (Seoul, KR) ; CHO; Jung
Joo; (Hwaseong-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Samsung SDI Co., Ltd. |
Yongin-si, Gyeonggie-do |
|
KR |
|
|
Family ID: |
56150890 |
Appl. No.: |
15/538949 |
Filed: |
July 27, 2015 |
PCT Filed: |
July 27, 2015 |
PCT NO: |
PCT/KR2015/007784 |
371 Date: |
August 18, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0525 20130101;
Y02E 60/10 20130101; H01M 2300/0025 20130101; H01M 10/052 20130101;
H01M 10/0567 20130101; H01M 10/0569 20130101; H01M 10/0568
20130101 |
International
Class: |
H01M 10/0567 20100101
H01M010/0567; H01M 10/0568 20100101 H01M010/0568; H01M 10/0569
20100101 H01M010/0569; H01M 10/0525 20100101 H01M010/0525 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2014 |
KR |
10-2014-0185909 |
Claims
1. An electrolyte solution for a lithium secondary battery, the
electrolyte solution comprising a lithium salt and an organic
solvent, wherein the electrolyte solution further comprises a solid
salt with an ammonium-based cation represented by Formula 1 and an
azide anion (N.sub.3--): ##STR00003## wherein, in Formula 1,
R.sub.1 to R.sub.4 are each independently hydrogen, a halogen, or a
C1 to C8 alkyl group.
2. The electrolyte solution of claim 1, wherein the solid salt is
at least one selected from the group consisting of ammonium azide,
tetramethylammonium azide, tetraethylammonium azide,
tetrapropylammonium azide, tetrabutylammonium azide,
tetrahexylammonium azide, tetraheptylammonium azide,
ethyltrimethylammonium azide, triethylmethylammonium azide,
butyltrimethylammonium azide, diethyldimethylammonium azide, and
dibutyldimethylammonium azide.
3. The electrolyte solution of claim 1, wherein an amount of the
solid salt is in a range of about 0.01 part to about 5 parts by
weight with respect to 100 parts by weight of a total weight of the
lithium salt and the organic solvent.
4. The electrolyte solution of claim 1, wherein the lithium salt
comprises at least one anion selected from the group consisting of
F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-, NO.sub.3.sup.-,
N(CN).sub.2.sup.-, BF.sub.4.sup.-, ClO.sub.4.sup.-, PF.sub.6.sup.-,
(CF.sub.3).sub.2PF.sub.4.sup.-, (CF.sub.3).sub.3PF.sub.3.sup.-,
(CF.sub.3).sub.4PF.sub.2.sup.-, (CF.sub.3).sub.5PF.sup.-,
(CF.sub.3).sub.6P.sup.-, CF.sub.3SO.sub.3.sup.-,
CF.sub.3CF.sub.2SO.sub.3.sup.-, (CF.sub.3SO.sub.2).sub.2N.sup.-,
(FSO.sub.2).sub.2N.sup.-, CF.sub.3CF.sub.2(CF.sub.3).sub.2CO.sup.-,
(CF.sub.3SO.sub.2).sub.2CH.sup.-, (SF.sub.5).sub.3C.sup.-,
(CF.sub.3SO.sub.2).sub.3C.sup.-,
CF.sub.3(CF.sub.2).sub.7SO.sub.3.sup.-, CF.sub.3CO.sub.2.sup.-,
CH.sub.3CO.sub.2.sup.-, SCN.sup.-, and
(CF.sub.3CF.sub.2SO.sub.2).sub.2N.sup.-.
5. The electrolyte solution of claim 1, wherein the organic solvent
is at least one selected from the group consisting of an ether, an
ester, an amide, a linear carbonate, and a cyclic carbonate.
6. The electrolyte solution of claim 1, wherein the electrolyte
solution comprises at least one selected from the group consisting
of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene
carbonate, a cyclic sulfide, a saturated sultone, an unsaturated
sultone, and a non-cyclic sulfone.
7. A lithium secondary battery comprising the electrolyte solution
according to claim 1.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to an electrolyte solution
for a lithium secondary battery, and a lithium secondary battery
including the electrolyte solution, and more particularly, to an
electrolyte solution that may improve high-temperature retention
characteristics of a lithium secondary battery, and a lithium
secondary battery including the electrolyte solution.
BACKGROUND ART
[0002] Along with technical development and increasing demand for
mobile devices, demand for secondary batteries as an energy source
is rapidly increasing. In particular, currently lithium secondary
batteries having high energy density, high working voltage, long
cycle lifetime, and low self-discharge rate are widely commercially
available.
[0003] Such a lithium secondary battery has a structure with an
electrolyte assembly impregnated with an electrolyte solution
containing a lithium salt, the electrolyte assembly including a
positive electrode and a negative electrode that are obtained by
coating electrode current collectors with respective positive and
negative active materials and are separated from one another by a
porous separator. During charging, lithium ions released from the
positive active material are intercalated into a negative active
material layer. During discharging, the lithium ions released from
the negative active material layer are intercalated into the
positive active material. The electrolyte solution serves as a
migration medium of lithium ions between the negative electrode and
the positive electrode.
[0004] In general, an electrolyte solution may include an organic
solvent and an electrolyte salt. For example, a widely used
electrolyte solution may consist of a mixed solvent of a
high-dielectric cyclic carbonate such as propylene carbonate, or
ethylene carbonate, and a low-viscosity chain carbonate such as
diethylcarbonate, ethylmethylcarbonate, or dimethylcarbonate, and a
lithium salt such as LiPF.sub.6, LiBF.sub.4, LiClO.sub.4 added to
the mixed solution.
[0005] Lithium-containing halide salts such as a lithium-containing
fluoride salt or a lithium-containing chloride salt which may be
used as the electrolyte salt are highly sensitive to moisture,
generating a strong acid (HX, wherein X is F, Cl, Br, or I) by
reaction with moisture during the manufacture of a battery or with
moisture present in the battery. In particular, since LiPF.sub.6 as
a lithium salt is unstable at high temperature, its anions may be
thermally decomposed, generating an acidic material such as
hydrofluoric acid (HF). This acidic material may unavoidably
accompany an undesirable side reaction within the battery.
[0006] For example, a solid electrolyte interphase (SEI) layer on a
surface of the negative electrode may be vulnerable to damage due
to strong reactivity of the hydrofluoric acid (HF), which may
induce continuous regeneration of the SEI layer, and increase the
thickness of the coated SEI layer of the negative electrode and an
interfacial resistance of the negative electrode. As lithium
fluoride (LiF) as a byproduct of the generation of hydrofluoric
acid (HF) is adsorbed onto the surface of the positive electrode,
an interfacial resistance of the positive electrode may also be
increased. In addition, the strong acid (HX) may cause radical
oxidation reaction within the battery, and consequently dissolution
and degeneration of the electrode active materials. In particular,
as transition metal cations included in a lithium metal oxide used
as a positive active material are dissolved, the cations may be
electrodeposited onto the negative electrode, forming an additional
coating layer on the negative electrode, consequently further
increasing the resistance of the negative electrode.
[0007] The SEI layer, which may be formed on the surface of the
negative electrode by reaction of a polar non-aqueous carbonate
solvent with lithium ions of the electrolyte solution during
initial charging of a lithium secondary battery, may serve as a
protective layer by inhibiting decomposition of the carbonate
electrolyte solution to stabilize the battery. However, an SEI
layer formed of only an organic solvent and a lithium salt may not
be enough to consistently serve as a protective layer and thus may
be gradually damaged during continuous charging and discharging of
the battery or storage of the battery at high temperature in a
fully charged state, due to increased electrochemical energy and
heat energy. A side reaction of decomposing a surface of the
negative active material exposed through the damaged SEI layer by
reaction with the electrolyte solution solvent may continuously
occur, leading to deterioration in characteristics of the battery,
including capacity reduction, lifetime reduction, and resistance
increase. Such a side reaction may generate gas in the battery. As
such gas generation is continued, the internal pressure of the
lithium secondary battery may be increased at high temperature,
causing swelling of the battery with increased thickness and
finally raising a safety concern of the battery.
[0008] To address these drawbacks, Patent document 1 (JP
2002-313415) discloses a non-aqueous electrolyte solution including
about 0.5 wt % to about 1.5 wt % of biphenyl and 0.5 wt % to about
2.0 wt % of cyclohexyl benzene (CHB) as additives. According to
this disclosure, swelling of a battery in a thickness direction may
be reduced when the disclosed battery is left even at high
temperature. A high-temperature characteristic test performed by
measuring a thickness of the battery after it was maintained at a
high temperature for 2 days supports that swelling of the battery
in the thickness direction was slightly suppressed, but not any
battery characteristic improvement effect. In particular, when the
storage period at high temperature is continued for a longer time,
battery capacity and capacity retention may be markedly reduced
even with using the additives.
PRIOR ART DOCUMENT
[0009] [Patent Document]
[0010] (Patent document 1) JP2002-313415 A
DETAILED DESCRIPTION OF THE INVENTION
Technical Problem
[0011] The present invention provides an electrolyte solution for a
lithium secondary battery that includes a solid salt with an
ammonium-based cation and an azide anion as an additive, thereby
improving high-temperature storage characteristics.
[0012] The present invention provides a lithium secondary battery
including the electrolyte solution.
Technical Solution
[0013] According to an aspect of the present invention, an
electrolyte solution for a lithium secondary battery includes a
lithium salt and an organic solvent, wherein the electrolyte
solution further includes a solid salt with an ammonium-based
cation represented by Formula 1 and an azide anion (N.sub.3--):
##STR00001##
[0014] wherein, in Formula 1, R.sub.1 to R.sub.4 are each
independently hydrogen, a halogen, or a C1 to C8 alkyl group.
[0015] In some embodiments, an amount of the solid salt may be in a
range of about 0.01 part to about 5 parts by weight with respect to
100 parts by weight of a total weight of the lithium salt and the
organic solvent. The solid salt may be at least one selected from
the group consisting of ammonium azide, tetramethylammonium azide,
tetraethylammonium azide, tetrapropylammonium azide,
tetrabutylammonium azide, tetrahexylammonium azide,
tetraheptylammonium azide, ethyltrimethylammonium azide,
triethylmethylammonium azide, butyltrimethylammonium azide,
diethyldimethylammonium azide, and dibutyldimethylammonium
azide.
[0016] According to another aspect of the present invention, a
lithium secondary battery includes the electrolyte solution.
Advantageous Effects of the Invention
[0017] As described above, according to the one or more
embodiments, an electrolyte solution may further include a solid
salt with an ammonium-based cation represented by Formula 1 and an
azide anion as an additive. A lithium secondary battery including
the electrolyte solution may have improved high-temperature storage
characteristics. Therefore, the lithium secondary battery may have
good capacity retention and discharge capacity even after
high-temperature storage for a long time.
DESCRIPTION OF THE DRAWING
[0018] FIG. 1 is a graph of open circuit voltage with respect to
high-temperature storage time in lithium secondary batteries
manufactured using electrolyte solutions of Example 1, Comparative
Example 1, and Comparative Example 2.
MODE OF THE INVENTION
[0019] Reference will now be made in detail to embodiments,
examples of which are illustrated in the accompanying drawings,
wherein like reference numerals refer to like elements throughout.
In this regard, the present embodiments may have different forms
and should not be construed as being limited to the descriptions
set forth herein. Accordingly, the embodiments are merely described
below, by referring to the figures, to explain aspects. As used
herein, the term "and/or" includes any and all combinations of one
or more of the associated listed items. Expressions such as "at
least one of," when preceding a list of elements, modify the entire
list of elements and do not modify the individual elements of the
list.
[0020] According to an aspect of the present disclosure, an
electrolyte solution for a lithium secondary battery includes: a
lithium salt and an organic solvent, wherein the electrolyte
solution further includes a solid salt with an ammonium-based
cation represented by Formula 1 and an azide anion (N.sub.3--):
##STR00002##
[0021] wherein, in Formula 1, R.sub.1 to R.sub.4 are each
independently hydrogen, a halogen, or a C1 to C8 alkyl group.
[0022] In the electrolyte solution according to an embodiment,
since the solid salt used as an additive has a lower reduction
potential than a carbonate organic solvent of the electrolyte
solution, the solid salt may be reduced at the surface of the
negative active material layer earlier than the organic solvent of
the electrolyte solution during initial charging of a battery,
forming a strong, dense SEI layer. Accordingly, the SEI layer
formed from the solid salt may prevent a side reaction such as
co-intercalation of the organic solvent of the electrolyte solution
into the negative active material layer or decomposition of the
organic solvent on the surface of the negative electrode, thus
improving lifetime characteristics of the battery. Furthermore,
since the SEI layer is a passivation layer having low chemical
reactivity, the SEI layer may exhibit high stability even during
longer cycles, and provide long lifetime characteristics.
[0023] A conventional lithium secondary battery may undergo sudden
performance deterioration in particular under high-temperature
environments, due to radical collapse of the SEI layer on the
surface of the negative electrode, consequently increased side
reaction between the negative electrode and the electrolyte
solution, increased generation of gas from decomposition reaction
of the electrolyte solution, and increased thickness (resistance)
of the negative electrode.
[0024] On the contrary, when the electrolyte solution according to
an embodiment is used, the SEI layer collapsed at high temperature
may be rapidly regenerated at a low potential due to the solid salt
used as an additive and may be continuously maintained, reducing
side reaction between the negative electrode having increased
reactivity due to such as high temperature or the like, and
providing improved general performance and high-temperature
characteristics of the battery.
[0025] The amount of the solid salt may be from about 0.01 part to
about 5.0 parts, and in some embodiments, about 0.1 part to about
3.0 parts by weight, with respect to 100 parts by weight of a total
weight of the lithium salt and the organic solvent.
[0026] When the amount of the solid salt is less than 0.01 part by
weight, it may be difficult to form a SEL layer having good
stability. On the other hand, when the amount of the solid salt
exceeds 5.0 parts by weight, charge and discharge efficiency may be
reduced.
[0027] In some embodiments, the solid salt may be at least one
selected from the group consisting of ammonium azide,
tetramethylammonium azide, tetraethylammonium azide,
tetrapropylammonium azide, tetrabutylammonium azide,
tetrahexylammonium azide, tetraheptylammonium azide,
ethyltrimethylammonium azide, triethylmethylammonium azide,
butyltrimethylammonium azide, diethyldimethylammonium azide, and
dibutyldimethylammonium azide. However, embodiments are not limited
thereto.
[0028] In some embodiments, a concentration of the lithium salt in
the electrolyte solution may be in a range of about 0.6M to about
2.0M, and in some embodiments, a range of about 0.7M to about 1.6M.
When the concentration of the lithium salt is less than 0.6M, the
electrolyte solution may have reduced conductivity and deteriorated
performance. On the other hand, when the concentration of the
lithium salt exceeds 2.0M, the electrolyte solution may have
increased viscosity, consequently leading to reduced mobility of
lithium ions. Any lithium salt commonly used in an electrolyte
solution for a lithium secondary battery may be used. For example,
anions of the lithium salt may be one selected from the group
consisting of F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-, NO.sub.3.sup.-,
N(CN).sub.2.sup.-, BF.sub.4.sup.-, ClO.sub.4.sup.-, PF.sub.6.sup.-,
(CF.sub.3).sub.2PF.sub.4.sup.-, (CF.sub.3).sub.3PF.sub.3.sup.-,
(CF.sub.3).sub.4PF.sub.2.sup.-, (CF.sub.3).sub.5PF.sup.-,
(CF.sub.3).sub.6P.sup.-, CF.sub.3SO.sub.3.sup.-,
CF.sub.3CF.sub.2SO.sub.3.sup.-, (CF.sub.3SO.sub.2).sub.2N.sup.-,
(FSO.sub.2).sub.2N.sup.-, CF.sub.3CF.sub.2(CF.sub.3).sub.2CO.sup.-,
(CF.sub.3SO.sub.2).sub.2CH.sup.-, (SF.sub.5).sub.3C.sup.-,
(CF.sub.3SO.sub.2).sub.3C.sup.-,
CF.sub.3(CF.sub.2).sub.7SO.sub.3.sup.-, CF.sub.3CO.sub.2.sup.-,
CH.sub.3CO.sub.2.sup.-, SCN.sup.-, and
(CF.sub.3CF.sub.2SO.sub.2).sub.2N.sup.-.
[0029] The organic solvent in the electrolyte solution may be any
organic solvent commonly used in an electrolyte solution for a
lithium secondary battery. For example, the organic solvent may be
an ether, an ester, an amide, a linear carbonate, or a cyclic
carbonate, which may be used alone or a combination of at least two
thereof.
[0030] Of these organic solvents, a cyclic carbonate, a linear
carbonate, or a carbonate compound as a mixture of the forgoing two
may be used. For example, the cyclic carbonate compound may be one
selected from the group consisting of ethylene carbonate (EC),
propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene
carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate,
vinylene carbonate, and a halide thereof, or a mixture of at least
two thereof. For example, the linear carbonate compound may be one
selected from the group consisting of dimethyl carbonate (DMC),
diethyl carbonate (DEC), dipropyl carbonate, ethylmethylcarbonate
(EMC), methylpropyl carbonate, and ethylpropyl carbonate, or a
mixture of at least two thereof. However, embodiments are not
limited thereto.
[0031] In particular, as cyclic carbonate organic solvents,
ethylene carbonate and propylene carbonate which have high
viscosity and high dielectric constant to dissociate a lithium salt
in electrolyte may be used. For example, an electrolyte solution
having a high electric conductivity prepared by mixing such a
cyclic carbonate with a linear carbonate having low viscosity and
low dielectric constant in an appropriate ratio may be used.
[0032] The ether as an organic solvent may be one selected from the
group consisting of dimethyl ether, diethyl ether, dipropyl ether,
methylethyl ether, methylpropyl ether, and ethylpropyl ether, or a
mixture of at least two thereof. However, embodiments are not
limited thereto.
[0033] The ester as an organic solvent may be one selected from the
group consisting of methyl acetate, ethyl acetate, propyl acetate,
methyl propionate, ethyl propionate, propyl propionate,
.gamma.-butyrolactone, .gamma.-valerolactone, .gamma.-caprolactone,
.sigma.-valerolactone, and .epsilon.-caprolactone, or a mixture of
at least two thereof. However, embodiments are not limited
thereto.
[0034] In some embodiments, the electrolyte solution for a lithium
secondary battery may further include a conventionally known
additive to form an SEI layer. For example, the additive to form an
SEI layer may be vinylene carbonate, vinyl ethylene carbonate,
fluoroethylene carbonate, a cyclic sulfite, a saturated sultone, a
unsaturated sultone, or a noncyclic sulfone, which may be used
alone or in a combination of at least two thereof. However,
embodiments are not limited thereto.
[0035] The cyclic sulfite may be, for example, ethylene sulfite,
methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethyl
ethylene sulfite, 4,5-diethyl ethylene sulfite, propylene sulfite,
4,5-dimethyl propylene sulfite, 4,5-diethyl propylene sulfite,
4,6-dimethyl propylene sulfite, 4,6-diethyl propylene sulfite, or
1,3-butylene glycol sulfite. The saturated sultone may be, for
example, 1,3-propane sultone, 1,4-butane sultone, or the like. The
unsaturated sultone may be, for example, ethene sultone,
1,3-propene sultone, 1,4-butene sultone, or 1-methyl-1,3-propene
sultone. The noncyclic sulfone may be, for example, divinylsulfone,
dimethyl sulfone, diethyl sulfone, methylethyl sulfone, or
methylvinyl sulfone.
[0036] The additive to form an SEI layer may be used in an
appropriate amount, which may vary depending on a type of the
additive. For example, the additive to form an SEI layer may be
about 0.01 part to about 10 parts by weight with respect to 100
parts by weight of the electrolyte solution.
[0037] According to another aspect of the present disclosure, a
lithium secondary battery includes an electrolyte solution
according to any of the above-described embodiments.
[0038] The lithium secondary battery may be manufactured by
injecting an electrolyte solution according to any of the
above-described embodiments into an electrode assembly including a
positive electrode, a negative electrode, and a separator between
the positive electrode and the negative electrode.
[0039] The positive electrode and the negative electrode may each
be manufactured by preparing a slurry by mixing positive or
negative active material, a binder, and a conducting agent, coating
a current collector such as aluminum foil with the slurry, and
drying and pressing a resulting product.
[0040] The positive active material may be a lithium-containing
transition metal oxide, for example, one selected from the group
consisting of Li.sub.xCoO.sub.2 (wherein 0.5<x<1.3),
Li.sub.xNiO.sub.2 (wherein 0.5<x<1.3), Li.sub.xMnO.sub.2
(wherein 0.5<x<1.3), Li.sub.xMn.sub.2O.sub.4 (wherein
0.5<x<1.3), Li.sub.x(Ni.sub.aCo.sub.bMn.sub.c)O.sub.2
(wherein 0.5<x<1.3, 0<a<1, 0<b<1, 0<c<1,
a+b+c=1), Li.sub.xNi.sub.1-yCo.sub.yO.sub.2 (wherein
0.5<x<1.3, 0<y<1), Li.sub.xCo.sub.1-yMn.sub.yO.sub.2
(wherein 0.5<x<1.3, 0.ltoreq.y.ltoreq.1),
Li.sub.xNi.sub.1-yMn.sub.yO.sub.2 (wherein 0.5<x<1.3,
0.ltoreq.y<1), Li.sub.x(Ni.sub.aCo.sub.bMn.sub.c)O.sub.4
(wherein 0.5<x<1.3, 0<a<2, 0<b<2, 0<c<2,
a+b+c=2), Li.sub.xMn.sub.2-zNi.sub.zO.sub.4 (wherein
0.5<x<1.3, 0<z<2), Li.sub.xMn.sub.2-zCo.sub.zO.sub.4
(wherein 0.5<x<1.3, 0<z<2), Li.sub.xCoPO.sub.4 (wherein
0.5<x<1.3), and Li.sub.xFePO.sub.4 (wherein 0.5<x<1.3),
or a mixture of at least two thereof. The lithium-containing
transition metal oxide may be coated with a metal such as aluminum
(Al) or a metal oxide thereof. In addition to these
lithium-containing transition metal oxides, a sulfide, a selenide,
or a halide of metal may be used.
[0041] The negative active material may be, for example, a
carbonaceous material, a lithium metal, silicon, or tin from which
lithium, ions may generally intercalated and deintercalated. For
example, the negative active material may be a metal oxide having a
potential less than 2V with respect to lithium, such as TiO.sub.2
or SnO.sub.2. Examples of the carbonaceous material may include
low-crystalline carbon and high-crystalline carbon. Examples of the
low-crystalline carbon may include soft carbon and hard carbon.
Examples of the high-crystalline carbon may include natural
graphite, artificial graphite, Kishgraphite, pyrolytic carbon,
mesophase pitch based carbon fiber, meso-carbon microbeads,
mesophase pitches, and high-temperature sintered carbon such as
petroleum or coal tar pitch derived cokes.
[0042] The binder attaches the active material to the conducting
agent and fixes them on a current collector. The binder may include
binders generally used in a lithium ion secondary battery. Examples
of the binder are polyvinylidene fluoride, polypropylene,
carboxymethyl cellulose (CMC), polyvinylpyrrolidone,
tetrafluoroethylene, polyethylene, polyvinyl alcohol, and
styrene-butadiene rubber.
[0043] The conducting agent may be, for example, artificial
graphite, natural graphite, acetylene black, ketjen black, channel
black, lamp black, thermal black, conductive fiber such as carbon
fiber or metallic fiber, conductive metal oxides such as titanium
oxide, and metallic powder such as aluminum powder or nickel
powder.
[0044] Examples of the separator are a single olefin such as
polyethylene (PE) and polypropylene (PP), or an olefin composite
thereof, polyamide (PA), polyacrylonitrile (PAN), polyethylene
oxide (PEO), polypropylene oxide (PPO), polyethylene
glycoldiacrylate (PEGA), polytetrafluoroethylene (PTFE),
polyvinylidene fluoride (PVdF), and polyvinylchloride (PVC).
[0045] The lithium secondary battery according to an embodiment may
have any shape not limited to a specific shape. For example, the
lithium secondary battery may be a cylindrical (can) type, a
rectangular type, a pouch type, or a coin type.
[0046] One or more embodiments of the present disclosure will now
be described in detail with reference to the following examples.
However, these examples are only for illustrative purposes and are
not intended to limit the scope of the one or more embodiments of
the present disclosure.
[0047] <Preparation of Electrolyte Solution>
Example 1
[0048] Ethylene carbonate, diethylcarbonate, and dimethylcarbonate
were mixed in a volume ratio of about 2:4:4 to prepare an organic
solvent. Next, LiPF.sub.6 as a lithium salt was dissolved in the
organic solvent to obtain a 1.15M LiPF.sub.6 mixture solution.
Next, 0.5 parts by weight of vinylene carbonate and 0.5 parts by
weight of tetrabutylammonium azide as a solid salt, with respect to
100 parts by weight of the LiPF.sub.6 mixture solution, were added,
thereby preparing an electrolyte solution.
Comparative Example 1
[0049] An electrolyte solution was prepared in the same manner as
in Example 1, except that no solid salt was added.
Comparative Example 2
[0050] An electrolyte solution was prepared in the same manner as
in Example 1, except that 1.0 part by weight of lithium
difluorophosphate was added, instead of 0.5 parts by weight of
tetrabutylammonium azide.
[0051] <Manufacture of Battery>
[0052] LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2 as a positive active
material, polyvinylidene fluoride (PVdF) as a binder, and carbon
black as a conducting agent were mixed in a weight ratio of about
91.5:4.4:4.1, and then dispersed in N-methyl-2-pyrrolidone to
prepare positive active material slurry. This slurry was coated on
an aluminum current collector, dried, and roll-pressed to
manufacture a positive electrode.
[0053] Graphite having a thickness of about 1 mm was used as a
negative electrode.
[0054] Next, the manufactured positive electrode, the negative
electrode, and a porous polyethylene membrane (available from
Tonen) as a separator were assembled into a coin cell, and a
corresponding electrolyte solution prepared as above was injected
thereinto.
[0055] <Evaluation Method>
[0056] 1. Cell Formation
[0057] The coin cells prepared by using the electrolytes of Example
1 and Comparative Example 1 were left at a constant temperature of
25.degree. C. for 12 hours, charged under conditions including a
constant current of 0.1 C until a voltage was 4.3 V and a constant
voltage having a cutoff current of about 0.05 C, and discharged
under conditions including a constant current of about 0.1 C until
a voltage was 3.0 V by using a lithium secondary battery
charger/discharger (TOSCAT-3600, available from Toyo-System Co.,
LTD), thereby completing a cell formation process.
[0058] 2. Lifetime Characteristics
[0059] After the cell formation, an initial discharge capacity (A1)
of each cell was measured after charging at room temperature
(25.degree. C.) with a constant current of about 0.2 C until a
voltage of about 4.3V was reached, and then with a constant voltage
with a cutoff voltage of about 0.05 C, and discharging with a
current of about 0.2 C until a voltage of 3.0V was reached.
[0060] 3. High-Temperature Storage Characteristics
[0061] To evaluate high-temperature storage characteristics at a
fully charged state of about 4.3V, each cell after the initial
discharge capacity (A1) measurement was charged with a constant
current of about 0.2 C until a voltage of 4.3V was reached and then
with the constant voltage with a cutoff current of about 0.05
C.
[0062] Each cell fully charged with 4.3V was maintained at about
60.degree. C. for about 6 days, and variations in open circuit
voltage (OCV) with respect to days of storage were measured. The
results are shown in FIG. 1. The OCV was measured after the cell
temperature was sufficiently cooled down.
[0063] To compare capacity reduction caused by high-temperature
storage, after each cell maintained at high temperature (60.degree.
C.) for 6 days was discharged at room temperature (25.degree. C.)
with a constant current of about 0.2 C until a voltage of 3.0V was
reached, a discharge capacity (A2) was measured. A capacity
retention was calculated using the following equation. The results
are shown in Table 1.
Capacity retention [%]=(Discharge capacity (A2) after storage at
60.degree. C. for 6 days/Initial discharge capacity
(A1)).times.100
[0064] Next, to evaluate a capacity recovery of each cell, the cell
after the discharge capacity measurement was recharged with a
constant current of 0.2 C until a voltage of 4.3V was reached and
with a constant voltage with a cutoff voltage of about 3.0V, and
then a discharge capacity (A3) of the cell was measured. A capacity
recovery was calculated using the following equation. The results
are shown in Table 1.
Recovery [%]=[Discharge capacity (A3) of cell recharged after
high-temperature storage/Initial discharge capacity
(A1)].times.100
TABLE-US-00001 TABLE 1 Before high- After high-temperature storage
(60.degree. C.) temperature storage Capacity retention Capacity
recovery Initial discharge Discharge Charge Discharge capacity (A1)
capacity (A2) Retention capacity capacity (A3) Retention Example
(mAh/g) (mAh/g) (%) (mAh/g) (mAh/g) (%) Example 1 160.02 126.1 78.8
138.4 140.7 87.9 Comparative 152.04 100.8 66.3 115.6 118.7 78.1
Example 1 Comparative 153.43 111.0 72.3 124.9 127.2 82.9 Example
2
[0065] Referring to Table 1, the coin cell manufactured using the
electrolyte solution of Example 1 had a higher discharge capacity
and a higher capacity recovery than those of the coin cells
manufactured using the electrolyte solutions of Comparative
Examples 1 and 2.
[0066] Referring to FIG. 1, the coin cell including the electrolyte
solution of Example 1 was found to have a less reduction in OCV
with respect to time, compared to the coin cells including the
electrolyte solutions of Comparative Examples 1 and 2. The less
reduction in OCV of the coin cell including the electrolyte
solution of Example 1 is attributed to the formation of a more
stable SEI layer due to the use of the additive, and effective
suppression of side reaction even at high temperature between the
electrolyte solution and lithium ions intercalated in the negative
electrode.
[0067] It should be understood that embodiments described herein
should be considered in a descriptive sense only and not for
purposes of limitation. Descriptions of features or aspects within
each embodiment should typically be considered as available for
other similar features or aspects in other embodiments.
[0068] While one or more embodiments have been described with
reference to the figures, it will be understood by those of
ordinary skill in the art that various changes in form and details
may be made therein without departing from the spirit and scope as
defined by the following claims.
* * * * *